Discrete energy levels and superconductivity in nanometer-scale Al particles

Abstract

We have fabricated single-electron tunneling transistors in which the central island is an aluminum grain with radius in the range 2-10 nm. The corresponding spacing between electron-in-a-box levels is in the range ~0.01 to ~1 meV. Using tunneling spectroscopy at 50 mK, we have, for the first time, resolved these discrete levels in a metallic grain. By observing the Zeeman spin splitting in a magnetic field, we can distinguish grains with even vs. odd numbers of electrons. A superconducting energy gap can be seen if the grain is large enough so that the level spacing is smaller than the energy gap. This gap is reduced continuously to zero by a magnetic field of 3 - 4 Tesla. While the superconducting gap adds to the Coulomb gap in determining the threshold voltage for tunneling into a grain with an initially even number of electrons, it subtracts from the Coulomb gap for a grain with an initially odd number of electrons because the tunneling electron can pair with the odd electron, forming a lower-energy fully-paired state.